Sediment dynamics in tropical highland catchments of central Mexico using fallout radionuclides
Résumé
Tropical regions are affected by intense soil erosion. This land degradation leads to important on-site (e.g. decrease in soil fertility) and off-site (e.g. reservoir siltation, water pollution) impacts. This study determined the mean soil particle and sediment residence times in soils and rivers of three catchments (3 – 12 km2) with contrasted land uses (i.e. cropland, forests, rangelands) located in highlands of the transvolcanic Mexican belt. Calculations were based on rainfall amount and river discharges as well as on fallout radionuclide measurements (Be-7, Cs-137, Pb-210) conducted on rainfall precipitated samples, soil sampled in the catchments and suspended sediment collected by automatic samplers in the river during most storms recorded all throughout the 2009 rainy season. Calculations using a radionuclide two-box balance model showed that the mean residence time of sediment in soils ranged between 5,000 (± 1,500) – 23,300 (± 7,000) years. In contrast, sediment residence time in rivers was much shorter, fluctuating between 50 (± 30) and 200 (± 70) days. The shortest mean residence times were measured in a hilly catchment dominated by cropland and rangelands, whereas they were the longest in an undulating catchment dominated by forests and cropland. Calculation of the Be-7/excess-Pb-210 in both rainfall and sediment allowed gaining insight on sediment dynamics throughout the rainy season. The first heavy storms of the year exported the bulk of the sediment stock accumulated in the river channel during the previous year. Our results show the priority of stabilising old gully systems and to concentrate the implementation of on-site erosion control measures in cropland and rangeland of the most reactive catchments.